1. Field of the Invention
This invention relates to a device for magnetically detecting the rectilinear displacement or the rotation angle of a subject, and in particular to a magnetic detection device provided with a plurality of magnetic tracks. This device can be applied broadly for detecting the rotation angle of a motor, for detecting the rotational speed, for positioning or speed control for automatic machines as well as for business machines.
2. Description of the Prior Art
A device for magnetically detecting the rotation angle or speed of an electric motor is disclosed in U.S. Pat. No. 4,274,053 (Susumu Ito et al.). According to the invention disclosed in this U.S. patent a magnetic recording medium 4 is mounted on an extremity of the shaft 2 of a rotary body 1 such as a rotor and magnetic signals are recorded therein, as indicated in FIG. 1. A magnetic sensor 6 consisting of a magneto-resistance element made of e.g. permalloy is disposed on a mounting plate 3, interposing a small gap therebetween, whereby the rotation angle of the rotary body 1 is detected as variations in resistance of the magneto-resistance element. In this magnetic detection device, there are problems due to magnetic leakage between different tracks in the magnetic recording medium, in the case where the magnetic recording medium 4 has a plurality of tracks and a multichannel magnetic rotation sensor 6 is used. For example, in the example shown in FIG. 2, magnetic leakage is produced, as indicated by arrows, between a track 4a and another track 4b in the magnetic recording medium 4. By this reason, the tracks 4a and 4b are subjected to mutual magnetic interference and thus it becomes impossible to detect correct signals recorded in each of the tracks.
In order to avoid this drawback, it is also conceivable not to dispose the tracks 4a and 4b so closely, but to locate them, interposing a gap large enough therebetween. However, in this case, this gives rise to another disadvantage that the magnetic recording medium 4 and the magnetic sensor 6 disposed in opposition to it are too large.